Wet-process orthophosphoric acid is commonly produced by treating phosphate rock with a strong mineral acid, usually sulfuric acid. Wet-process orthophosphoric acid which contains about 55 weight percent P.sub.2 O.sub.5 has a boiling point of about 300 degrees Fahrenheit and is a viscous liquid in which all of the phosphate is in the ortho form. There are many disadvantages associated with the use, transportation and storage of ortho-phosphoric acid. For example, there are significant storage and transportation cost associated with ortho-phosphoric acid because of the high water content. Furthermore, liquid fertilizers of high P.sub.2 O.sub.5 analysis cannot be made with orthophosphoric. Because of these disadvantages, orthophosphoric acid is usually converted into superphosphoric acid, i.e. phosphoric acid containing at least about 60 weight percent of the total P.sub.2 O.sub.5 in the polyphosphate form.
There are basically three methods that have been used either commercially or semi-commercially to evaporate wet-process orthophosphoric acid from its normal commercial strength to higher ortho-content of superphosphoric acid. At about 67% P.sub.2 O.sub.5, wet process phosphoric acid becomes more or less an anhydrous form, and as the concentration is increased by water loss above about 67% above-noted, the water comes from condensing or polymerizing the phosphoric acid to poly-phosphates.
One mode of converting orthophosphoric acid into superphosphoric acid is boiling. As orthophosphoric acid is boiled, it is concentrated by evaporation of water, forming superphosphoric acid. During the boiling procedure, the orthophosphoric acid becomes more viscous. Currently in the United States of America, the largest amount of superphosphoric acid (acid containing polyphosphate) is produced in recirculating vacuum evaporators heated with high pressure steam or hot oil. The equipment is made of the most corrosion-resistant metal alloys available. The concentration of the product from these evaporators is dependent on the temperature and pressure (vacuum) that the equipment can tolerate or generate--the product strength is independent of the strength of acid used as a feed, although 54% P.sub.2 O.sub.5 is normally used. Many years of experience indicate that these evaporators can operate only over a very narrow range, i.e. 68-71% total P.sub.2 O.sub.5 and 10%-40% poly-phosphate in the product. There are several disadvantages associated with the production of superphosphoric acid by concentration of orthophosphoric acid through boiling. For example, boiling orthophosphoric acid is highly corrosive and its handling during concentration presents a number of difficult problems because of the presence of fluorine containing compounds, its highly corrosive properties, its high viscosity and the like.
A second type evaporator which has been used only semi-commercially and not at all at the present time, is the submerged combustion evaporator. A necessary feature of submerged combustion is that the reactor vessel must at all times contain a pool of the liquid being evaporated and hot gases are introduced well under the surface of the pool. The evaporation occurs from the pool and the concentrated product is withdrawn from the pool. This is the type of evaporation taught by the Getsinger Pat. No. 3,317,306 (U.S. Patent). The submerged combustion process was used for several years during the mid to late 1960's by Occidental Chemical Co., USS Agricultural Chemical Co., and Swift company. Occidental operated the largest plant, and for the longest time; that company published literature and sold semicommercial quantities. That process failed for several reasons: (1) attempts to scale up to larger, more economical units failed; (2) the units plugged with solids rapidly due to long residence time of 10 to 30 minutes and had to be shut down and cleaned out during shut-down every two or three days. The frequent shutdowns coupled with a restart time of about 4 hours gave very poor production rates. The highest polyphosphate these units ever achieved was about 55% and by 1970 the market was demanding over 60% polyphosphate in a premium ammonium polyphosphate product which these units could not provide. This process of the submerged combustion for converting wet-process orthophosphoric acid into superphosphoric acid results in a total product of about 70% P.sub.2 O.sub.5 of which less than 55% is polyphosphate form as noted-above, and is disclosed in Volume 53, No. 9, September 1961 of Industrial and Engineering Chemistry. In this process, the wet-process orthophosphoric acid is introduced beneath the surface of a pool of the acid. Combustion products are also introduced into the pool of acid through a submerged burner. Superphosphoric acid containing a mixture of ortho and polyphosphoric acids is withdrawn at a fixed level above the pool of acid into an entrainment separator where acid mists are coalesced and the resultant superphosphoric acid flows by gravity to the product sump. Exit gases from the entrainment separator are scrubbed with water and the gas exiting from the scrubber are exhausted to the atmosphere. This submerged burner process of converting wet-process orthophosphoric acid into superphosphoric acid has several disadvantages. For example, start-up periods for this process are prolonged unduly. From 3 to 6 hours are required to attain equilibrium conditions to produce the desired strength of superphosphoric acid. Another disadvantage of this process is that insolubles are formed (due probably to the relatively long residence time of the acid within the pool and for also other reasons) which contaminate the superphosphoric acid product. Another disadvantage of this submerged combustion method is that the superphosphoric acid produced by the process invariably has such a high percentage fluoride content that it cannot be used as a raw material for the production of feed grade products. Still another disadvantage is that the concentration of the non-orthophosphoric acid component, i.e., the polyphosphates, in the product acid fluctuates as variations occur in the distribution of the hot gas within the pool of liquid being concentrated. Moreover, another disadvantage of the process is that the superphosphoric acid produced by the process when the concentration is effected at an acid pool temperature of about 500 degrees Fahrenheit, contains significantly large amounts of the orthophosphoric acid, usually about 50% of the total P.sub.2 O.sub.5 in the acid product, whereby polyphosphate content is normally about 50% or less.
A third type commercial evaporator to produce superphosphoric acid is the hot gas entrainment as described by Esteroy in U.S. Pat. No. 3,671,202. Only subsequent to the filing of the Esteroy U.S. patent did it become known to produce commercial ultra-filtered orthophosphoric acid that thereby feed-acid can have a commercial strength greater than about 50% to about 55% P.sub.2 O.sub.5 orthophosphoric acid content. An incorrect assumption would be that P.sub.2 O.sub.5 polyphosphate content of a product is directly proportional to P.sub.2 O.sub.5 concentration of feed orthophosphoric acid; to the contrary, the concentration of the polyphosphate product from evaporators of the type used for the process of Esteroy, is completely independent of strength of the feed orthophosphoric acid used. In fact, the product concentration (% of total P.sub.2 O.sub.5 weight content) in resulting superphosphoric acid is dependent upon the temperature that the equipment can tolerate or generate; even Esteroy patent acknowledges this, that it is dependent upon temperature. This type process, such as of Esteroy, is a process in which the wet-process orthophosphoric acid is fed continuously into an evaporation zone where it is contacted with heated gases having a temperature of from 600 degrees Fahrenheit to about 1800 degrees Fahrenheit, preferably from about 700 to 1400 degrees Fahrenheit. The amount and velocity of the gas is controlled relative to the amount of wet-process orthophosphoric acid so that the acid is entrained in the gas stream in the form of droplets. The resultant hot gas stream carrying the entrained particles of acid and water vapor is passed continuously through a second zone where the liquid droplets are coalesced and the desired liquid superphosphoric acid is withdrawn. The lean gases, i.e., the heating gases from which the superphosphoric acid has been separated, are quenched, scrubbed with water and the uncondensed gases leaving the scrubber disposed of in any desired manner, as for example, by discharge into the atmosphere. This type process also suffers from a number of inherent disadvantages. For example, while the patent states generally that an acid product in which up to 72 weight percent of the P.sub.2 O.sub.5 is in the form of superphosphoric acid that can be prepared, other patents have indicated that experimentation has demonstrated that in actual fact the weight percent of superphosphoric acid in the product is considerably lower. Still another disadvantage associated with this process is that increased sold deposition occurs which results in frequent stops in production so that solids can be removed from process hardware. Yet another disadvantage of this process is that fluorides are emitted into the atmosphere which could possibly result in an adverse impact on the environment. The Esteroy patent illustrates and discloses that utilizing a process step-wise substantially similar to that of the present invention, the percentage of total P.sub.2 O.sub.5 of a superphosphoric acid product will be increased within a product-range of from about 74% total P.sub.2 O.sub.5 (from a feed having 50% (Ex.I) and 53.5% respectively of P.sub.2 O.sub.5) to 80% (allegedly) total P.sub.2 O.sub.5 (from a feed of orthophosphoric acid having (allegedly) 74% total P.sub.2 O.sub.5 by weight. Estroy patent at-length points-out that the product bears a direct proportional relationship to an increase in termerature of reaction--the higher the temperature in the vapor space, the greater the non-ortho proportion of P.sub.2 O.sub.5. It is pertinent to point out that not only does Esteroy repeatedly state this fact, that it is solely with regard to increasing the temperature to increase the total percentage as well as the non-ortho percentage of P.sub.2 O.sub.5, there is nothing to the contrary nor about any other way of making such increase, nor whether such increase is the sole answer; the sole feeds disclosed or mentioned in that patent are the above-noted Examples I and II thereof at 50 percent and 53.5 percent respectively.
Getsinger U.S. Pat. No. 3,317,306 relates to the submerged combustion evaporator in which the reactor vessel contains a pool of liquid being evaporated by virtue of hot gases as above noted, such being totally different and distinct and in no way related to the present invention nor to the method of the Esteroy patent, being totally non-analagous.
Young U.S. Pat. No. 3,044,855 is unrelated to the present invention, that patent disclosing solely (and nothing more) that addition of ammonium ion prevents "foaming" in a type of process employing the unrelated above-noted submerged combustion evaporator of the type disclosed by Getsinger patent above noted. Prevention of foaming in a submerged combustion evaporation method has heretofore constituted a major concern, the Young patent having nothing to do with ammoniating of a superphosphoric acid in a method that does not even relate to a submerged combustion evaporator. Foaming is not a consideration in the process of Esteroy patent.
Only in years subsequent to the filing of the Esteroy patent did it eventually become known to produce commercial ultra-filtered orthophosphoric acid that thereby can have a commercial strength greater than about 50 to 55% P.sub.2 O.sub.5 content. Moreover, there is no prior art with regard to orthophosphoric acid of such higher concentration of P.sub.2 O.sub.5.